GHCi 9.2.0.20210422 erases left hand side of interactive declaration when tab completion is requested
## Summary
When pressing <kbd>⭾</kbd> while in an interactive prompt, all entered text left of the last equals sign (`=`) is removed.
## Steps to reproduce
1. Open GHCi (`ghci` or `ghc --interactive`)
2. Type some text containing an equals sign (`=`) such as `let x = 2 +`
3. Press the “tab” key (<kbd>⭾</kbd>)
## Expected behavior
GHCi will attempt to autocomplete the word left of the cursor.
## Actual behavior
GHCi performs the expected behavior but also erases all text left of or at the rightmost equals sign (`=`).
## Demonstration
View this behavior in action in this terminal recording:
[https://asciinema.org/a/424970](https://asciinema.org/a/424970)
## Environment
* GHC version used: 9.2.0.20210422
Optional:
* Operating System: Pop!_OS 21.04 on Linux 5.11.0-7620-generic
* System Architecture: x86 64
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